Recently, there has been an increasing interest in looking into Orthogonal Frequency Division Multiplexing (OFDM) transmission methods for wireless cellular systems. OFDM is normally associated with wireless Local Area Network (LAN) systems (e.g., IEEE 802.11a) or Digital Video Transmission (DVB-T). Two drivers behind the use of OFDM for wide-bandwidth (and often high data rate systems) are:    a. The Fourier transform-based modulation of OFDM which allows for the use of simple equalizers such as Zero-Forcing (ZF) or Linear Minimum Mean Squared Error (LMMSE) in OFDM receivers, and    b. Inherent frequency diversity in OFDM systems in frequency-selective channels as a result of multicarrier modulation.
The frequency diversity in the OFDM systems can be exploited by encoding the data across frequency subcarriers. Another method to take advantage of the inherent frequency diversity is to spread data symbols across subcarriers using a set of spreading codes as in Direct Sequence Code Division Multiple Access (CDMA) systems. This method, usually referred to as MultiCarrier CDMA (MC-CDMA), can benefit from the potential frequency diversity and improve the raw performance of the system, provided that a Maximum Likelihood (ML) receiver or a sufficiently good suboptimal receiver is used. Systems based on combination of a CDMA scheme and OFDM signaling have been reported. A wideband multicarrier system has been proposed by multiplexing a number of narrowband systems, and different receiver algorithms have been studied for these types of systems.
Automatic Repeat reQuest (ARQ) is a technique that can guarantee reliable communication especially at low SNR, where Forward Error Correction (FEC) coding alone may not be able to provide the required performance. ARQ can be used alone in an uncoded system (i.e., standard ARQ) or together with FEC coding (i.e., hybrid ARQ). A number of different ARQ algorithms have been proposed for OFDM and MC-CDMA systems. In one technique, a maximal ratio combining frequency diversity ARQ scheme is proposed for OFDM systems, in which a different interleaving pattern is used for each retransmission. In another technique, two hybrid ARQ algorithms have been proposed for MC-CDMA systems. In the first algorithm, constellation mapping is rearranged before each retransmission, whereas in the second algorithm, a rearrangement of spreading code assignment is carried out before retransmissions.